I am considering having a basement dug out under my existing house and into the rear of the property in London and wondered about other people experience with contractors. Did you get what you needed with/without an Architect? Just a Contractor? Any Recommendations? Positive experience?

I would get an architect involve as 90% of the time you will need excavation & this could quite well involved work on your existing foundations. Also see how deep you water table is & also see if drainage can connect straight to the main sewers/soak away without involving pumps.

From what I have read, this type of extension is quite popular in Chelsea and Kensington, with exotic basements including gyms and swimming pools.
This is a specialist area, you need to find someone with an excellent track record.

Safety standards in many basement projects are well short of acceptable, as our inspection initiative shows. Keep in mind that all building and fire codes must be followed during the basement construction process.

No doubt, For this kink of work we have to consult with a well experienced contractor as it is the best solution and is really helpful to have a quality work.The nature of this work is very sensitive and if we want to finish this work with a proper ending we have to combine our forces with an expert..